No-relationship between impossibility of faster-than-light quantum communication and distinction of ensembles with the same density matrix

Chuan Wang, Gui Lu Long, Yang Sun

Published 2005-06-06, updated 2006-04-20Version 2

It has been claimed in the literature that impossibility of faster-than-light quantum communication has an origin of indistinguishability of ensembles with the same density matrix. We show that the two concepts are not related. We argue that: 1) even with an ideal single-atom-precision measurement, it is generally impossible to produce two ensembles with exactly the same density matrix; or 2) to produce ensembles with the same density matrix, classical communication is necessary. Hence the impossibility of faster-than-light communication does not imply the indistinguishability of ensembles with the same density matrix.